Flat metallic particles of controlled thickness and shape typically are difficult and expensive to make. Ball milling results in relatively thick flake with little control of size and shape. Vacuum deposition followed by a chemical or mechanical removal of particulates from the substrate is costly with little control of shape of the flake. Vacuum/chemical vapor deposition of coatings onto existing particulates such as flakes (e.g., mica) or spheres produces flakes limited to the shape and size of the existing particulate. Pre-existing flake shapes are typically too thick and the shape too jagged for high performance coatings.
Thin film metal particulates are expensive, because existing process to make them, like those described in U.S. Pat. No. 4,879,140 or 5,100,599, use exotic equipment such as plasma generators or vacuum chambers, or are labor intensive, small scale processes like photolithography. The equipment cost and relative slow rate of production using skilled labor to operate the sophisticated equipment increases the cost The prior art particulates are not readily produced in reasonable volume, and cost as much as $5,000/oz. At these prices, paints that use the particulates as the pigment are only suitable for highly specialized applications. There is a need for a lower cost, higher volume process for rapidly and reliably making thin film metal particulates usable as paint pigments.
In U.S. Pat. No. 5,100,599, Jensen et al. described a vapor deposition/photo-lithography method for making thin film particulates of controlled shape. Independent deposition sites are defined with layers of photoresist. The thin films are deposited on the sites. Then, the photoresist is dissolved to free the flakes.
In U.S. Pat. No. 5,895,524, Dickson described a method for making thin film metal particulates including the steps of immersing a metallized sheet of fluorinated ethylene propylene (FEP) first in an aqueous base and then in an aqueous acid to loosen and release the metal from the FEP. The particulates are brushed from the FEP into the acid tank, and are recovered. The FEP is reusable. The particulates are usually aluminum or germanium metal having a thickness of about 900 to 1100 Å, and preferably, 1000 Å. The method for freeing the particulates may also include ultrasonically vibrating the metallized sheet following the immersions.
For making aluminum particulates, the preferred base is 7% Na2CO3 and the preferred acid is 0.01–0.1 N acetic acid. For making germanium particulates, the preferred base is 2.5 N NaOH, since this metal is harder to loosen from the FEP. The acid bath neutralizes the basic reaction between the metal film and base.
The base immersion takes about 15 seconds. Prior to the acid immersion, the base-treated metallized film is exposed to air for about 25 seconds. The acid immersion lasts about 15 seconds before brushing the particulates from the FEP. A metallized roll of the FEP is readily towed through the several operations in a continuous process, as will be understood by those of ordinary skill.
Particulates are recovered from the acid bath by filtering, rinsing, and drying. The particulates are sized. Then, as described in U.S. Pat. No. 5,874,167, the particulates are treated using conventional aluminum treatments. Suitable treatments include applying chemical conversion coatings or protective sol coatings. The conversion coatings may be chromic acid anodizing, phosphoric acid anodizing, Alodine treating (particularly using either Alodine 600 or Alodine 1200); cobalt-based conversion coating as described in Boeing's U.S. Pat. Nos. 5,298,092; 5,378,293; 5,411,606; 5,415,687; 5,468,307; 5,472,524; 5,487,949; and 5,551,994; or the like. The sol coating method creates a sol-gel film on the surface using a hybrid organozirconium and organosilane sol as described in Boeing's U.S. Pat. No. 5,849,110. Related sol-gel coated aluminum flakes are described in U.S. Pat. No. 5,261,955.
The different treatments can impart different tint to the pigment. Alodine imparts a yellow or greenish-yellow tint. The cobalt treatments impart blue tints.
The sol coating is preferable a hybrid mixture wherein the zirconium bonds to the aluminum flake covalently while the organic tail of the organosilane bonds with the paint binder. The anodizing treatments promote adhesion primarily by mechanical surface phenomena. The sol coating provides adhesion both through mechanical surface phenomena (surface microroughening) and through chemical affinity, chemical compatibility, and covalent chemical bonds.
The particulates are pigments for paints or surface coatings and generally are used in urethane, cyanate ester, or urea binders. The organosilane in the sol coating generally will include a lower aliphatic amine that is compatible with the binder.
Kenneth Suslick of the University of Illinois pioneered research into sonochemistry, a technique that uses the energy of sound to produce cavitation bubbles in a solvent. The bubbles collapse during the compression portion of the acoustic cycle with extreme microscale energy release evidenced by high (microscale) localized temperatures and pressures estimated at about 5200° F. and 1800 atm, respectively. Suslick determined that sonochemistry was an effective way to produce amorphous metal particles. He developed laboratory processes for making amorphous iron agglomerates desired as catalysts in hydrocarbon reforming, carbon monoxide hydrogenation, and other reactions.
Suslick also discovered that he could produce metal colloids and supported catalysts if he sonicated the metal precursors (principally volatile metal carbonyls or other organometallics) with a suspended polymer like polyvinylpyrrolidone or with suspended inorganic oxide supports, such as silica or alumina.
Suslick's work focused on sonochemical techniques to form catalysts composed of agglomerated metal nanoparticles. These catalysts are efficient because of their large surface areas. His work is described in the following articles that we incorporate by reference:                (1) K. Suslick, “Sonochemistry,” 247 Science 1439–1445 (23 Mar. 1990);        (2) K. Suslick et al., “Sonochemical Synthesis of Amorphous Iron”, 353 Nature 414–416 (3 Oct. 1991); and        (3) K. Suslick, “The Chemistry of Ultrasound,” Yearbook of Science & the Future, Encyclopedia Britannica, Inc., 138–155 (1994).Similar work is described in the following articles by Lawrence Crum, that we also incorporate by reference:        (1) L. Crum, “Sonoluminescence,” Physics Today, September 1994, pp. 22–29, and        (2) L. Crum “Sonoluminescence, Sonochemistry, and Sonophysics”, J. Acoust. Soc. Am. 95(1), January 1994, pp. 559–562.        
Gibson sonicated Co2+ (aq) with hydrazine to produce anisometric cobalt nanoclusters. Science, vol. 267, Mar. 3, 1995. He produced anisometric, hexagonal disk-shaped, cobalt nanoclusters about 100 nanometers in width and 15 nanometers in thickness with oriented (001) crystals comparable to cells of α-cobalt. The nanoclusters were small enough to be strongly influenced by Brownian forces and thereby were resistant to agglomeration. Working with hydrazine, however, on a commercial scale poses safety questions.
In U.S. Pat. Nos. 5,520,717; 5,766,764; and 5,766,306, Boeing described a process to create “nanophase” or “nanoscale” amorphous metal particles with Suslick's sonochemistry techniques using organometallic precursors like iron pentacarbonyl (Fe(CO)5) in an alkane (like n-heptane or n-decane) under an inert atmosphere with sonication at about 20 kHz and 40–100 Watts for 0.1–24 hours. The particles (distributed in the range of about 5–100 nm in diameter) were extracted from the alkane using a polar solvent of reasonably high vapor pressure, such as ethylene glycol monomethyl ether (CH3O—CH2CH2—OH). Then, a polymer or polymeric precursors (especially those of vinylpyrrolidone, an acrylic, or a urethane) were added with or without surfactants to coat and separate the metal particles.
To produce individual or agglomerated metal particles in the particle size distribution range of 10–30 nm with sonochemistry, a continuous process involved the steps of:                (a) feeding neat metal carbonyl, like iron pentacarbonyl, to a reactor;        (b) sonicating the neat metal carbonyl to produce nanoscale particles; and        (c) separating the particles from the metal carbonyl, preferably in a magnetic separator.This process produced nominal 30 nm diameter particles of iron or iron alloys. Being continuous eliminated the need to use an alkane or water, and, thereby, greatly simplified the process. Using the hydrocarbon impaired the continuous preparation of the particles when we attempted larger reaction quantities and tried to replenish the reactants, although we do not understand why the production rate declined when a hydrocarbon medium was used in addition to the organometallic precursor (i.e. Fe(CO)5). To avoid undue agglomeration and to produce finer particles smaller than 30 nm in diameter, a surfactant was added prior to separation of the particles.        
Agglomerated particles from such a process can be reconstituted into a large individual particle by rapidly heating the particles with, for example, microwaves to the melt followed by resolidification into a unitary nanophase particle. Generally these nanoscale particles are smaller than are practical for our preferred coatings.
In U.S. Pat. No. 3,419,901, Nordblom described a method for producing flakes of nickel about 1/16 inch square by about 0.000040 inches (1 μm) thick. Nordblom applied an electrically nonconducting grid over a cathode and plated nickel. He removed the nickel plate as flakes by impinging sprays of electrolyte or other fluids on the cathode. The flakes were used in nickel-alkaline batteries along with nickel oxyhydrate active material to increase conductivity of the positive plates.
Nordblom described that a prior art process to Pilling (U.S. Pat. No. 2,365,356) deposited nickel directly on a stainless steel cathode to produce a highly strained deposit of sheet nickel. This sheet broke up naturally into flake and sloughed off. Such flakes tended to curl and were unacceptable for batteries because of their shape. Also, they were too thick.
Nordblom suggested using a stainless steel or chrome-plated steel cylinder or drum scored with grooves 0.020 inches in depth to define the flakes. The drum was disposed with its axis extending substantially horizontally so that a portion of the drum's surface would dip into the electrolyte bath. Epoxy resin filled the grooves on the drum to create a grid and to define individual areas for growth of flakes, similar to the deposition sites Jensen used with the photolithography techniques described in U.S. Pat. No. 5,100,599. Nordblom plated the nickel from a nickel sulfamate bath and knocked the flakes from the drum using a stream-of water or electrolyte. Generally, Nordblom metallographically and electrically polished (in phosphoric, sulfuric, and chromic acid) the surface of the electrode.